What is it?
Infection with Marteilia sydneyi, also known as QX disease and marteiliosis, is a a protozoan parasite of the phylum Paramyxea that affects Sydney rock oyster.
Where and when might it occur?
Outbreaks usually occur in summer and autumn. Warm temperatures favour parasite development, leading to greatest mortality at the end of summer. Oysters may be subjected to infection for a period of only two weeks per year. The disease is associated with low salinity and high water temperature.
Transmission is horizontal; the pathogen passes from the environment into the epithelium of the palps and gills where it proliferates without forming spores. The life cycle of M. sydneyi is not fully understood. Intermediate hosts are required to complete the life cycle; most recent studies confirm that a benthic polychaete is an intermediate host.
Diagnosis
The disease often results in high cumulative mortalities of up to 100 percent.
Gross pathological signs are:
- shrunken body
- generally poor condition
- death within 60 days of infection
- colourless and translucent tissues because the gonad is completely resorbed
- digestive gland (usually a deep green or brown colour) becomes pale yellow brown
Microscopic pathological signs are:
- focal haemocytic inflammation of the gills
- epithelial and conepithelial and connective tissue hyperplasia of the gill epithelium, with fusion of gill filaments
Source: Australian Government, Department of Agriculture, Fisheries and Forestry